Metal thin films are formed on the surfaces of building materials, vehicle parts and the like in order to impart a favorable appearance or a sense of high quality. Known examples of methods for forming metal thin films include vapor deposition and sputtering. In addition, chrome and chrome alloys are frequently used as metals for forming a metal thin film, and allow the obtaining of a superior appearance in the sense of high quality.
A coating film is normally provided in the form of a topcoat on the surfaces of such metal thin films, and serves to protect the surface of the metal thin film or impart an attractive appearance thereto.
Known coatings for forming a coating film consist of heat-curable powder coating compositions and heat-curable liquid agent-type coatings.
For example, Patent Document 1 discloses a method for providing a topcoat layer by coating a urethane-based topcoat coating onto a metal thin film layer of a base material composed of a soft resin material on which is sequentially provided a base coat layer and a metal thin film layer, and then heat curing the topcoat coating by baking.
In addition, Patent Document 2 discloses a method for providing a topcoat layer by baking a urethane-based topcoat coating incorporating a silane coupling agent having epoxy groups onto a metal thin film layer of a base material composed of a soft resin material on which is sequentially provided a base coat layer and a metal thin film layer.
Moreover, Patent Document 3 discloses a method for forming a transparent resin coating film (topcoat) by baking a coating composed of an acrylic resin and a melamine resin onto a thin film in a metal or resin material having a resin coating film formed on the surface of a metal or resin material, a thin film of a titanium alloy having a chrome-plated appearance formed on the resin coating film, and a transparent resin coating film formed in the form of a protective layer on the thin film.
However, in a method in which a coated film is formed by baking a coating on a metal thin film as described in Patent Documents 1 to 3, a considerable amount of time for coating work was required, thereby resulting in low productivity.
Therefore, coatings (active energy beam-curable coatings) have been proposed that contain active energy beam-curable components such as ultraviolet ray-curable components, which are cured with the progression of a polymerization reaction induced by ultraviolet light, since they enable work time to be shortened and have superior mechanical properties.
However, active energy beam-curable coatings were subject to the occurrence of curing shrinkage of the coating film (cured product) when cured by being irradiated with an active energy beam such as ultraviolet light. The occurrence of curing shrinkage causes cracks to form in metal thin films coated by the coating film, resulting in increased susceptibility to a decrease in adhesion between the coating film and the metal thin film. Susceptibility to decreased adhesion was particularly great in the case of metal thin films formed by vapor deposition or sputtering.
As an example of a coating that solves the aforementioned problems, Patent Document 4 discloses a photocurable coating composition for metal vapor deposition top coating that contains an acrylic polymer, a radial polymerizable compound containing urethane acrylate, for example, and a photoinitiator. In addition, a method is also described for curing a coating composition by coating the photocurable coating composition for metal vapor deposition top coating onto a metal vapor deposition surface followed by irradiating with ultraviolet light.
However, in the photocurable coating composition for metal vapor deposition top coating described in Patent Document 4, although initial adhesion and scratch resistance were favorable, there were cases in which the composition was unable to impart superior water resistance, rust prevention and alkaline resistance to a metal thin film.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H9-156034    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H11-5270    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2002-219771    [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2006-169308